Processing semiconductor rods



United States Patent-O PROCESSING sEMrcoNDUcToR RoDs Theodor Rmmel, Munich, Wolfgang Keller, Pretzfeld, Schloss, and Hans-Friedrich Quast, Munich, Germany, assignors to Siemens & Halske Aktiengesellschaft Berlin and Munich, a corporation of Germany Application April 13, 1959, Serial No. 806,174 Claims priority, application Germany April 22, -1958 20 Claims. (Cl. 219-10.43)

This invention relates to the processing of semiconductor rods and is particularly concerned with automatic thickness regulation incident to zone drawing applied to semiconductor rods.

C. Theurer has described a method of zone drawing of semiconductor rods, especially silicon rods, comprising holding a silicon rod at its opposite ends, and drawing a molten zone produced by inductive heating through the medium of a high frequency coil, axially of the rod. Such method is employed for purifying semiconductor material and for producing mono-crystalline semiconductor rods.

The invention proposes a method of and a device for producing incident to zone drawing a mono-crystalline semiconductor rod of defined thickness, that is, of a thickness which remains uniform within a desired range.

In accordance with the invention, the current fed from a high frequency current source to the coil surrounding the semiconductor rod, which changes with changing thickness of the rod, is utilized for controlling the operation of a device adapted to change the spacing between the holding means for the semiconductor rod, such device moving the holding means respectively toward and away from each other, until the current flowing in the high frequency coil again assumes the desired value.

The high frequency coil is in accordance with the invention very short as compared with the length of the semiconductor rod. It is advantageous but not absolutely necessary to employ the high frequency coil for the control of the thickness of the semiconductor rod at the same time for the heating of the molten zone. If desired, a separate coil supplied from a separate high frequency generator, may be used for the thickness regulation.

The various objects and features of the invention will be brought out in the course of the description which will be rendered below with reference to the accompanying drawings, in which Fig. l shows the principles underlying the operation;

Fig. 2 indicates the power N delivered by the generator depending upon the ratio of the load resistance ZH to the output resistance of the transmitter ZS; and

Fig. 3 illustrates a feature of the heating system.

In Fig. 1, numeral 1 indicates a semiconductor rod, for example, a silicon rod which is held at its ends 2 and 3. In the illustrated example, the high frequency coil 5 also serves for the heating of the zone 4. This entire arrangement is contained in a quartz tube (not shown) in a protective gas atmosphere. A supporting field coil may be provided for additionally increasing the molten zone. The supporting field coil and the high frequency coil 5 are arranged so that they can be moved along the rod at a given speed. For example, if the molten zone is moved -in the direction indicated by `the arrow 1,8, the device for compressing and stretching the rod during the zone drawing will in accordance with the invention be arranged at the end of the semiconductor rod indicated by numeral 2. The device is generally always arranged at the end ICC of the rod which solidities last because, in such a case, faults occurring in the crystal grid due to the compression and stretching of the rod can be eliminated by the successive melting zone, resulting in rods exhibiting good crystal perfection.

A- capacite-r 6 may be disposed in parallel with the high frequency coil 5 for compensating the inductive characcircuit. The coupling is thereby so loose that the system generator-heating circuit will oscillate with one frequency even responsive to a further deturning of the heating circuit caused by thickness variations of the semiconductor rod.

The high frequency current flowing in the coil 5 induces in the part of the semiconductor rod surrounded thereby, a voltage which produces a current the magnetic eld of which opposes the eld produced by the coil. This part of the semiconductor rod can therefore be L compared with a coil winding. As is known, the coupling factor of such coil arrangement is a function of the geometric dimensions of the coils and proportional to the diameter of the inner coil. Therefore, if the thickness of the semiconductor rod changes during the operation according to the invention, the coupling factor and therewith the resistance ZH of the heating` circuit, measured across the terminals 12, 13, will likewise change. This load variation effects a variation in the anode current which is indicated by the instrument 8. A definitely determined magnitude of the anode current will accordingly correspond to a definite thickness of the rod.

The variable yresistor 9 is at the start of the operation so adjusted that an anode current corresponding to the desired rod thickness causes a voltage drop at such resistor, which is equal to the voltage delivered by the battery 10, so that the polarized relay 11 willl be in a normal position with the contact spring 15 positioned as shown. If the thickness of the semiconductor rod and therewith the anode current of `the generator change in the course of the operation, the relay 11 will energize, causing the contact 15 to establish engagement with contact 16 or contact 17, depending upon the direction in which the current change took place. Closure of the circuit over one or the other of these contacts 16 or17 will effect operation of motor 21 to actuate by Way of a suitable gear (not shown) a toothed wheel 19 for the purpose of moving the ratchet 20 so as to effect either compression or stretching of the semiconductor rod depending upon the direction of rotation of the toothed 'wheel 19.

vbeing accordingly within the ascending branch of the curve.

It will be yappreciated from previous explanations that the coupling yfactor will become smaller for a rod becoming thinner, that is, vthe coupling between the high fre# quency coil and the part of the semiconductor represent- Patented Nov. 17, 1959 ing a coil Winding, will become looser and ZH will accordingly become greater. It is apparent from Fig. 2 that the matching will then improve, vthat is, the delivered power and therewith the anode current, will increase. However, when the semiconductor rod becomesl thicker, lresulting in tighter coupling, ZH and also the anode current will become smaller.

rIi'he load resistance ZH may also be such as to permit working within the descending branch of the curve, about at the point B, that is, the power N delivered by the generator and therewith the anode current will become smaller when the rod becomes thinner and ZH increases. In order to obtain thickness regulation in the desired direction, the relay 11 vmust be oppositely poled as compared with the operationV which utilizes the working point A in the ascending branch of the curve.

If the Working point is along the ascending branch of the curve, the anode current indicated by the instrument 8 will rise when the rod becomes thinner. The relay lil must be poled so that it closes, for example, the contact 16' to cause operation of the motor 21 so as to rotate the toothed wheel 19 in the direction of the arrow 22, resulting in compression'of the semiconductor rod, that is, in reducing the spacing between the ends 2 and 3 for an interval until the anode current has again the desired value, causing the relay 11 to restore with'the contact spring 15 in the normal position shown.

When the rod becomes thicker, the anode current will become lower and relay lll'will operate its contact i5 to engage contact 17, causing the motor 21 to rotate the toothed wheel 19 in the direction of the arrow 23, result* ing in stretching of the semiconductor rod until the anode current again assumes the desired Value to effect restoration of the relay'll. Y

The heating system shown in Fig. 3 is connected when it is desired to change the thickness of the semiconductor rod during the drawing. In addition to the high frequency coil 24 which surrounds the semiconductor rod 1, there is provided a second coil 25 in series therewith. The characteristic impedance of these two coils can again be compensated by means of a'eapacitor 26. The inductance of the coil 25 may be varied, for example, by immersion into or withdrawal from the coil 25, of a metallic memberV 27, thereby affecting the operatively effective load resistance of the system between the terminals 12 and 13 and therewith the Vanode current and the thickness of the semiconductor rod. i

Changes may be made within lthe scope and spirit of the appended claims which define what is believed to be new `and desired to have protected by Letters Patent'.` We claim: i

1. In the art of zone drawingl semiconductor material, wherein a rod of highly pure semiconductor material is held at its opposite ends by holding means therefor, 'and Ywherein a molten zone, produced by inductive Vheating through the medium of a high frequency coil surrounding 'said rod, is moved axially of said rod, Vthe improvement which consists in utilizing the current supplied'from a high frequency current source to thecoil surrounding said rod, which current changes with changing thickness of the rod, for controlling the operation of a device adapted to vary the spacing between said rod-holding means, said device being operative to move said rod-holding means `respectively together and apart to effect compression and stretching of the semiconductor rod until the current owingin said high frequency coil assumes a desired Y value.

coilsurrounding said rod, .said moltenkzolne beingrnoved axially of said'rod, a method of automatically regulating the thickness of the molten zone tormaintain the thickness of the rod substantially uniform, said method comprising utilizing the current supplied/to said high frequency coil from a high frequency current source, which current changes with changing thickness of said molten zone, for causing a force to De applied'to said rod-holding means so as to vary the spacing therebetween for the purpose of respectively stretching and compressing said semiconductor rod, thereby maintaining the molten Zone at a desired thickness and consequently causing a current to ow in said high frequency coil which is of a magnitude corresponding to the desired thickness of the semiconductor rod.

3. A method according to claim 2, wherein said high frequency coil is short as compared with the length of said semiconductor rod.

4. A method according to claim 2, wherein said force is applied at -the end of said semiconductor rod which solidiies last. Y

5. A method according to claim 2, wherein said high frequency coil is in circuitV with said high frequency current source to provide for increased current iow from said high frequency current source responsive to decreasing thickness of the molten zone.

6. A method according to claim 2, wherein said high frequency coil is in circuit with said high frequency current source to provide for decreased current ow from said high frequency current source responsive to decreasing thickness of said molten zone. i

7. A method according to claim 2, wherein a capacitor is disposed in parallel with sai'd high frequency coil.

8. A method according to claim 2, wherein the oscillating circuit of said high frequency current source is detuned with respect to the oscillating circuit of said high frequency coil, said circuits being relatively loosely coupled over a coupling member. Y

9. A method according to claim 2, comprising connecting a further coil in series with said high frequency coil and changing the inductance of said further coil by disn placement of a metallic member relative'thereto, for the purpose of changing the thickness of the semiconductor rod during the processing thereof.

10. In apparatus for processing highly pure semiconductor rods including means for holding a semiconductor rod to be processed at the opposite ends thereof and having a high frequency coil which surrounds said semiconductor rod and which is supplied With current from a high frequency source to effect melting of a zone along said rod b y inductive heating thereof,v which zone is moved axially of said rod, an Varl-rangement for automatically regulating the 4thickness of saidv semiconductor rod during the processing thereof, comprising a device for varying the spacing between said rod-holding means to'effect respectively stretching and compressing of said rod, operating means for said device,A control means for governing said operating means, and Vcircuit means controlled by the current supplied by said high frequency source to said high frequency coil, which current changes with changing thickness ofpsaid molten zone, for governing theractuation of said control means.

ll. A structure and cooperation of 'parts according to claim- 1'0, wherein said high frequency coil is short as compared with the length of 'said ser'nieonductor rod.

12j. VA structurenand cooperation ofparts according to claim 10, wherein said device for varyingthespaein-g between saidlrod-holding means cooperates*withl said rodat the end thereof which soliditles last. q l l v v 1.3. A structureandcooperation of parts according to claim 1.0, wherein said high frequency coilV ,is Vin circuit with saidY highl yfrequencyfcurrent .source to Vprovide for Aincreased current iiow from jsaidfsource responsive to decreasing thickness of said molten zone. I

with saidhigh frequency; current sourceto Vprovide for decreased current flow from said source responsive to decreasing thickness of said molten zone.

15. A structure and cooperation of parts according to claim 10, comprising a capacitor connected in parallel with said high frequency coil.

16. A structure and cooperation of parts according to claim 10, wherein the oscillating circuit of said high frequency current source is detuned with respect to the oscillating circuit of said high frequency coil, and a coupling member for relatively loosely coupling said oscillating circuits.

1.7. A structure and cooperation of parts according to claim l0, in combination with means for changing the thickness of the semiconductor rod during the processing thereof, comprising a further coil connected in series with said high frequency coil, and a metallic member disposed for immersion into and withdrawal from said further coil to change the inductance thereof.

18. A structure and cooperation of parts according to claim l0, wherein said control means comprises -a relay connected to the anode circuit of said high frequency current source for governing the actuation of said operating means to control the operation of said device to effect 6 compressing or stretching of said semiconductor rod responsive respectively to decrease and increase of the thickness of said molten zone.

19. A structure and cooperation of parts vaccording to claim 18, wherein said operating means comprises a reversible motor for controlling said device in two directions of motion thereof, and circuit means governed by said relay for controlling the directional rotation of said motor depending upon variations of the anode current of said high frequency current source.

20. A structure and cooperation of parts according to claim 19, comprising a capacitor and a resistor connected in series with said relay.

OTHER REFERENCES Dipl.-Phys. Reimer, German application Serial No. S. 39812, Vll1c/21g printed May 30, 1956 (kZlgGll 02). 

